There is a never ending search for advanced materials. Many of these advanced materials are composite materials. Composites are formed when various distinct materials are engineered together to create a new material. The idea is to take best advantage of the strengths of each component material complementing their weaknesses. Composites may be engineered with unique properties to suit very distinct applications such as solar cell, catalyst, sensor, or electronic device. An important property that has been frequently investigated is electrical conductivity. In different applications, there is always a strong demand to employ a composite material having superior electrical conductivity. One of the outstanding examples is dye sensitized solar cell (DSSC).
DSSC based on mesoporous TiO2 is a promising low-cost, high-efficiency photovoltaic device for solar energy conversion. To-date, a power conversion efficiency (PCE) of 15% has been obtained. Despite this, further improvement is necessary for DSSC to compete with silicon-based solar cells.
In a conventional DSSC, a monolayer of molecular dye is adsorbed at the surface of a mesoporous wide band-gap semiconductor oxide film, such as TiO2, or ZnO. However, electron transport in nanoparticle-based device is limited by a trap-limited diffusion process. The slow charge diffusion increases the probability of recombination, resulting in lower efficiency. Moreover, the grain boundaries encountered during electron transport lead to fast recombination prior to their collection at the electrode. Much effort has been devoted to improve charge transport property and collection efficiency.
One promising solution is to use a one-dimensional (1-D) nanostructure photoanode to replace the nanoparticle film, which provides a direct pathway for collection of charges generated throughout the device. Electron transport in 1-D nanostructures, such as nanowires, nanofibers, nanorice (elongated 1-D structure with two ends having smaller diameter compared to rest of the structure) or nanorods, is expected to be several orders of magnitude faster than that of nanoparticles. Another approach to improve the electron transport and collection is by incorporating highly electrically conductive materials, such as carbon tubes, graphite, in titanium photoanode. The presence of conductive materials in a titanium photoanode is expected to improve the charge transport properties and extend the electron lifetime, thereby improving the performance of the device. Several groups have reported that utilizing nanocomposite photoanodes, such as titanium/carbon nanotubes, and titanium/graphene, can enhance electron transport and collection efficiency.
Electron transport in two architectures of nanocomposites in prior arts are depicted in FIGS. 1A and 1B.
As shown in FIG. 1A, carbon nanotubes (CNTs) 101 are totally surrounded or embraced by TiO2 nanoparticles 102. At first, electron-hole pairs are firstly generated in the TiO2 nanoparticles 102 under light excitation, and the electrons 103 migrate towards the CNTs 101 from the TiO2 nanoparticles 102. Electron transfer routes 104 within and across the TiO2 nanoparticles 102 are shown by curly arrows. Once the electrons 103 arrive at the CNTs 101, acting as direct-charge transport superhighways, the electrons 101 are rapidly transferred towards fluorine doped tin oxide (FTO) glass 105 due to the superior conductivity of the CNTs 101. Direct electron transfer routes 106 within the CNTs 101 are shown by straight arrows.
As shown in FIG. 1B, the CNTs 101 are totally surrounded or embraced by TiO2 nanorods 107. Similarly, electron-hole pairs are firstly generated in the TiO2 nanorods 107 under light excitation, and the electrons 103 migrate towards the CNTs 101 from the TiO2 nanorods 107. The electron transfer routes 104 within and across the TiO2 nanorods 107 are shown by thinner straight arrows. Once the electrons 103 arrive at the CNTs 101, the electrons 103 are rapidly transferred towards the FTO glass 105 due to the superior conductivity of the CNTs 101. The direct electron transfer routes 106 within the CNTs 101 are shown by straight arrows.
However, regarding the two fore-mentioned architectures in the prior arts, the electron transfer routes for the generated electrons to travel from the generation sites to the CNTs are long and many electrons are susceptible to recombine with the holes (in case of solid-state DSSC) and electrolytes (in case of liquid DSSC) on their routes to the FTO glass electrode, leading to decrease of the power conversion efficiency. For the first architecture, the electrons are easily recombined with the holes in the hole transfer materials (HTM) for solid-state DSSC or oxidized electrolytes for the liquid DSSC at the grain boundaries of the TiO2 nanoparticles. Also, the TiO2 nanoparticles are loosely attached with each others, thus extending significantly the electron transfer routes. For the second architecture, the electrons are also easily recombined with the holes or electrolytes at the grain boundaries of the TiO2 nanorods and at exposed areas of CNT not fully covered with nanorods. Similarly, the TiO2 nanorods are loosely attached with each other, thus extending significantly the electron transfer routes.
Additionally, the TiO2 nanoparticles, or the TiO2 nanorods are loosely attached on the CNTs, leading to limited amount of electrical contact surface areas between the CNTs and the TiO2 nanoparticles or the TiO2 nanorods, thereby reducing electron transfer rate and increasing the exposure of CNT to the holes in HTM and oxidized electrolytes which increases also the probability of the electron-hole recombination.
Consequently, there is an unmet need for a nanocomposite which is effective in providing fast electric charge transport, and reducing the rate of electron-hole recombination, ultimately increasing power conversion efficiency.